1. Technical Field
This disclosure relates generally to image forming apparatuses, liquid discharging heads, image forming methods, recorded matter, and recording liquid.
2. Description of the Related Art
Generally, printers, facsimile machines, copiers, plotters, and image forming apparatuses in which plural functions of the aforementioned devices are combined in a single apparatus are configured to perform image formation (recording, printing, imaging, character printing are also used as synonyms). One method of image formation is performed by using a liquid discharging device including a recording head configured with a liquid discharging head that discharges (jets) liquid droplets of recording liquid (hereinafter, also referred to as “ink”). The recording liquid is caused to adhere to a medium (hereinafter, also referred to as a “sheet” made of any material; recording medium, transfer material, recording sheet are also used as synonyms) while the sheet is being conveyed.
An “image forming apparatus” refers to an apparatus that performs image formation by discharging (jetting) liquid onto media such as paper, threads, fiber, cloth, leather, metal, plastic, glass, wood, ceramics, etc. “Image formation” not only refers to forming images with some meaning such as characters or figures onto a medium but also forming images without meaning onto a medium, such as patterns. “Liquid” is not limited to recording liquid and ink; it can be any kind of fluid to be discharged such as a DNA sample, resist, and pattern material. A “liquid discharging device” is any device that discharges liquid through its liquid discharging head, and is not limited to a device for forming an image.
There are several issues to be addressed by an inkjet recording apparatus, which is an example of an image forming apparatus including such a liquid discharging device. The first issue is recording speed. In a so-called serial-type inkjet recording apparatus, a recording head that is narrower than the width of a recording sheet scans the sheet many times and discharges liquid to perform recording. This is a method of recording with “lines”, which is considerably disadvantageous in terms of recording speed compared to the electrophotographic printing method or the offset printing method of recording in units of pages, i.e., with “planes”.
In an attempt to resolve such a disadvantage in speed, the cycle of ejecting liquid droplets has been increased and the recording head has been enlarged, so that a larger area is recorded in each scanning operation and recording can be completed with fewer scanning operations. As a result of such an attempt, there is known a line-type inkjet recording apparatus employing line-type heads, in which nozzles are provided across the width of a sheet in such a manner as to cover the entire recording area of the sheet.
Patent Document 1: Japanese Laid-Open Patent Application No. 2004-074611
With this line-type inkjet recording apparatus, the sheet only needs to be passed through once to complete the recording operation. Accordingly, the recording speed is many times higher than that of conventional serial-type inkjet recording apparatuses, thus considerably eliminating the disadvantage in terms of speed.
The second issue pertains to cost of sheets (recording medium). By using so-called paper dedicated for inkjet printing, superior image reproducibility can be attained, with considerably high-quality recorded images that are comparable with photographs.
However, inkjet-dedicated paper is generally expensive, and therefore, offices subjected to severe cost cutting are hesitant to purchase such sheets. Furthermore, in cases of printing for commercial purposes where a large amount of printouts are required, it is extremely disadvantageous if high-quality images can be achieved only with inkjet-dedicated paper.
Accordingly, the composition of ink has been modified so that high-quality images can be achieved even with plain paper (copier paper) that is heavily used in offices. For example, there have been attempts to develop dye-based ink with low permeability, to use a fixing auxiliary agent, and to develop pigment-based ink. As a result, in recent years and continuing, the same level of image quality as that of a laser printer can be attained by using plain paper, i.e., copier paper, which is generally used in offices.
In image forming apparatuses employing an electrophotographic method, coloring material is heat-fixed onto sheets. However, in inkjet recording apparatuses, coloring material is fixed onto sheets with the use of permeation and moisture evaporation. The permeation process is accompanied by various problems and restrictions.
As described above, the quality of images recorded on plain paper has improved significantly. However, this does not apply to every type of sheet. In particular, on sheets that cannot sufficiently absorb ink, the overflowing unabsorbed ink often becomes solid and forms undesirable irregularities (beading phenomenon).
In order to jet ink onto such sheets, a conventional approach is to perform a multi-path recording method in which the recording operation is divided into plural paths, so that sufficient time can be spent on drying the ink. Even on a sheet with inferior absorbency, the overflowing ink can be fixed without solidifying by providing enough waiting time for moisture to evaporate and drying to progress.
However, in the line-type inkjet recording apparatus where recording speed is given high priority, it is difficult to provide enough waiting time for drying. Although it is possible to decrease the recording speed and the sheet conveying speed even in the line-type inkjet recording apparatus so as to provide sufficient drying time, this runs counter to the original purpose of the line-type inkjet recording apparatus of increasing the recording speed.
Patent document 2 discloses a technology for increasing the drying speed by heating the sheet. Specifically, there is provided a first drying unit arranged on the upstream side of line heads to heat-dry the recording medium from the side on which an image is not to be recorded (hereinafter, “non-recording side”). A second drying unit is arranged on the downstream side of the line heads to heat-dry the recording medium from the non-recording side after printing. A third drying unit is arranged facing the second drying unit to heat-dry the recording medium from the side on which the image is recorded in a non-contact manner.
Patent Document 2: Japanese Laid-Open Patent Application No. 2005-001303
Patent Document 3 discloses a line-type inkjet recording apparatus that performs multi-path recording. Specifically, a recording unit intermittently performs recording while the recording heads are moved to predetermined positions in an orthogonal direction and a recording sheet is conveyed backward and/or forward in the sheet conveyance direction. A procedure of conveying the sheet for a predetermined number of times is repeated for a predetermined number of times until an image is recorded on the entire recording sheet.
Patent Document 3: Japanese Laid-Open Patent Application No. 2005-205828
However, by installing a heating unit such as the heater described in Patent Document 2, the sheet may be curled due to the difference in expansivity on the side of the sheet with ink and the other side of the sheet without ink. If the sheet curls during the recording operation, the sheet may contact the heads. As a result, the sheet may be soiled, or in the worst case, the sheet may become caught in the heads and cause a paper jam that would stop the mechanism. Meanwhile, in the multi-path recording method described in patent document 3, the high speed of the line-type inkjet recording apparatus is disadvantageously decreased.